Gliomas are malignant brain tumors thought to arise through multi-step tumorigenesis, involving both the activation of oncogenes and the loss of tumor suppressor genes. The ros1 gene encodes a proto-oncogenic protein which has been implicated, by in vitro studies, in the pathogenesis of several types of cancer, including gliomas. Northern blot analysis revealed expression of ros1 mRNA in 3 (30%) of 10 primary glioma specimens. In situ hybridization localized ros1 mRNA transcripts to GFAP positive tumor cells and pericytes around capillaries. Immunohistochemistry using an antibody specific for ros1 demonstrated strong positivity amongst neoplastic glial cells in the same glioma samples. No ros1 mRNA or protein was detected in 5 normal brain specimens. These data provide the first evidence for the overexpression of ros1 mRNA and protein in primary human gliomas, and are consistent with a proposed oncogenic role of ros1 in these tumors.